Net Income per Unit (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of net (loss) income per unit [Abstract]
Net income - basic	$ 45,585	$ 36,159
Additional allocation of equity in net income attributable to AB resulting from assumed dilutive effect of compensatory options	355	296
Net income - diluted	$ 45,940	$ 36,455
Weighted average units outstanding - basic (in units)	100,548,000	96,114,000
Dilutive effect of compensatory options (in units)	1,098,000	1,077,000
Weighted average units outstanding - diluted (in units)	101,646,000	97,191,000
Basic net income per unit (in dollars per unit)	$ 0.45	$ 0.38
Diluted net income per unit (in dollars per unit)	$ 0.45	$ 0.38
Out-of-the-money options excluded from the diluted net income per unit (in units)	2,774,117	2,828,033